Among other uses, thin layers on a silicone base serve as surface coatings for the most varied materials. For example, paper and high-polymer substances can be coated and metals can be varnished with silicones. Silicone layers can also serve as primary coatings of lightwave guides (see in this connection, for instance: "Adhaesion", Vol. 29, (1985), No. 10, pages 28 to 35, as well as "Plaste and Kautschuk", Vol. 34, (1987) No. 5, Pages 183 to 190).
Unsaturated organosiloxanes are commonly polymerized and cross-linked thermally with the addition of suitable initiators such as peroxides or catalysts such as platinum compounds. The photochemical polymerization and cross-linking of unsaturated organosiloxanes through, for example, irradiation with light, especially UV light, is also known. Without the addition of photo initiators and/or photo sensitizers, however, long irradiation times are required for this purpose. Direct photo cross-linking of organosiloxanes without the addition of photo initiators or photo sensitizers requires the presence of chromophores in the molecule which enter into interaction with the light. Such structural elements are, for instance, maleinimide groups.
Non-functionalized organosiloxanes such as poly(dimethyl siloxane) and poly(methylphenyl siloxane) are discolored by irradiation with UV light; gel formation i.e., cross-linking takes place only in the presence of air (see: "Polymer Preprints", Vol. 20 (1979) No. 2, Pages 443 to 446). Upon irradiation with a low-pressure mercury lamp, poly(methylphenyl siloxane) only forms a superficial film, and only partial cross-linking takes place. After an irradiation of more than 100 hours, a change of the mean molecular weight of less than 2% is found (see: "J. Polym. Sci.-Part A-1", Vol. 4, (1966) pages 2107 to 2120).
However, the above-mentioned methods suffer the disadvantage of not producing high-purity and thermally stable thin films. In particular, impurities due to the required addition of photo initiators and photo sensitizers have a technically adverse effect. A further disadvantage is that upon UV cross-linking, discoloration, insufficient transparency and insufficient thermal stability result for thin layers made from silicone acrylates. A further disadvantage of the known methods is that they are not economical because long irradiation times are required (see in this connection, for instance,: "J. Organometal. Chem.", Vol. 148 (1978), Pages 213 to 223).
Accordingly, it is an object of the invention to provide a method for generating thin layers with a silicone base having high purity, thermal stability and good dielectric properties by photo hardening which requires only short irradiation times.